In recent years, there has been a growing demand for human body detection with the aim of offering security and safety in various social fields. In addition to counter-terrorism measures spreading worldwide, human detection is the most important problem for nursing care for the elderly and rescue operations after a disaster. Conventionally, there is a way to detect a human body by using an infrared device and a camera. However, there is a problem that a system using infrared ray has difficulty in distinguishing between human body temperature and an outside environment in a place where temperature is high. Moreover, there is a problem that a system using a camera faces an extreme fall in sensitivity in an environment where light cannot be sufficiently captured at night or during bad weather. In recent years, an imaging system using radar has drawn attention as a way to solve these problems.
However, there are many cases where imaging using radar requires many antennas or receivers in order to obtain sufficient data for estimating a shape. For example, a phased-array radar system obtains information about a shape of an object by transforming phases of many receivers (transmitters) and controlling and scanning directivity and a direction of radio waves. However, imaging using the radar leads to enlarging or complicating the system with a result that the system is expensive.
As a conventional way to solve the problems and detect an object and the object direction by a simple configuration, for example, there is a method using a technique known as Doppler and direction-of-arrival as disclosed in Non-Patent Literature 1.
FIG. 16 shows a configuration of a conventional radar apparatus.
A radar apparatus 901 shown in FIG. 16 includes a transmitter 910, receivers 920 and 930, a transmission antenna 911, and receiving antennas 912 and 913.
The radar apparatus 901 detects targets 931, 932, and 933. The radar apparatus 901 emits, from the transmitter 910, detection radio waves having a certain frequency and receives radio waves reflected from the targets 931, 932, and 933 via the receivers 920 and 930.
When the targets 931, 932, and 933 are moving at a certain radial velocity with respect to the radar apparatus 901, a frequency of reflected waves received by the receivers 920 and 930 shifts by a frequency corresponding to the radial velocity with respect to the frequency of the detection radio waves radiated from the transmission antenna 911. From the shifted frequency, a radial velocity for each of the targets 931, 932, and 933 can be detected.
Here, the radial velocity is a velocity component along a direction from the radar apparatus 901 to a target among velocities of the targets 931, 932, and 933. In this case, the radial velocity is a relative velocity component of each of the targets 931, 932, and 933 with respect to the radar apparatus 901. In other words, as shown in FIG. 16, assuming that the respective velocities of the targets 931, 932 and 933 are V1, V2, and Vi, the radial velocities for the targets 931, 932, and 933 are V1f, V2f, and Vif, respectively, which are velocities divided along the respective directions from the radar apparatus 901 to the targets 931, 932, and 933.
In other words, the radar apparatus 901 detects the radial velocities V1f, V2f, and Vif of the targets 931, 932, and 933, respectively, from the frequency of reflected waves received by the receivers 920 and 930 with respect to the frequency of the detection radio waves.
Incidentally, the radar apparatus 901, as shown in FIG. 16, has two systems each of which includes a receiving antenna and a receiver corresponding to the receiving antenna. Furthermore, the receiving antennas 912 and 913 are provided at different places.
With this, a distance from each of the targets 931, 932, and 933 to the receiving antenna 912 is mutually different from a distance from each of the targets 931, 932, and 933 to the receiving antenna 913.
In this way, it is possible to detect directions of the targets 931, 932, and 933 thanks to the difference in a distance from the targets 931, 932, and 933 to two receiving antennas 912 and 913. Hereafter, a principle of detecting directions will be described in detail.
In FIG. 16, for example, because the target 933 is nearer to the receiving antenna 913 than the receiving antenna 912, reflected waves from the target 933 reach the receiving antenna 913 earlier than the receiving antenna 912. When the reflected waves received by the receiving antenna 912 are compared with the reflected waves received by the receiving antenna 913, the reflected waves received by the receiving antenna 912 are delayed in phase compared with the reflected waves received by the receiving antenna 913. Here, assuming that the target 933 is in a direction of θi from the front surface of the receiving antennas 912 and 913 and that the two receiving antennas are provided at a distance d, a phase difference between the reflected waves received by the receiving antenna 912 and the reflected waves received by the receiving antenna 913 can be represented by Expression 1. It is noted that a phase of the reflected waves received by the receiving antenna 912 is φ1, a phase of the reflected waves received by the receiving antenna 913 is φ2, and a wavelength of the detection radio waves emitted from the transmission antenna 911 is λ.φ2−φ1=2πd sin θ/λ  (Expression 1)
When a transformation of Expression 1 results in Expression 2 as described below, it is possible to detect a direction θ of the target 933 from a phase difference φ2−φ1 in reflected waves received by the two receiving antennas 912 and 913.θ=sin−1{(φ2−φ1)λ/(2πd)}  (Expression 2)
This is a technique called direction-of-arrival (DOA).
As described above, the conventional radar apparatus 901 shown in FIG. 16 makes it possible, by detecting both a phase and a Doppler frequency, to identify a plurality of targets and detect a direction and a velocity for each of the targets. It is noted that in FIG. 16, the radar apparatus 901 cannot detect only a one-dimensional direction because the radar apparatus 901 has the two receiving antennas 912 and 913, but for example, can detect a horizontal-and-vertical two-dimensional direction by disposing one more receiving antenna other than on a straight line including the receiving antennas 912 and 913.
The radar apparatus 901, for example with respect to human detection, can detect a human body by using a different motion for each of the human body parts. Specifically, because the head, hands, and legs move at different velocities with respect to the trunk of the body, it is possible to detect a human body from the directions and the velocities.
Incidentally, as a conventional technique of detecting an object and the direction, Patent Literature 1 discloses a technique of detecting a direction of an object from a beam pattern property for each of the antennas and a delay time of a spreading code.